database medterm

• Entity


  An object that exists and is distinguishable from other objects
• Entity


  • Specific person, company, event, plant
• Entity set


  A set of entities of the same type that share the same properties
• Entity set


  • Set of all persons, companies, trees, holidays
• Attribute


  Descriptive properties possessed by all members of an entity set
• Attributes


  • ID, name, street, city, salary (for instructor entity set)
  • course_id, title, credits (for course entity set)
• Domain


  The set of permitted values for each attribute
• Attribute types


  • Simple and composite attributes
  • Single-valued and multivalued attributes
  • Derived attributes
• Simple attribute


  Attribute that consist of a single atomic value
• Simple attribute


  • Salary
• Composite attribute


  Attribute value not atomic
• Composite attributes


  • Address: 'House_no:City: State'
  • Name : 'First Name: Middle Name: Last Name'
• Single valued attribute


  Attribute that hold a single value
• Single valued attributes


  • City
  • Customer id
• Multi valued attribute


  Attribute that hold multiple values
• Multi valued attributes


  • A customer can have multiple phone numbers, email id's
  • A person may have several college degrees
• Derived attribute


  An attribute that's value is derived from a stored attribute
• Derived attribute


  • Age, derived from the stored attribute Date of Birth
• A relationship is an association among several entities
• Types of relationships


  • One-to-one relationship (1:1)
  • One-to-many relationship (1:M)
  • Many-to-many relationship (M:N)
• Cardinality constraints are expressed by drawing either a directed line (→), signifying "one," or an undirected line (-), signifying "many," between the relationship set and the entity set
• Cardinality constraints can also be expressed by numbering each entity. * or, m for many
• One-to-one relationship


  • A student is associated with at most one instructor via the relationship advisor
  • A student is associated with at most one department via stud_dept
• One-to-many relationship


  • An instructor is associated with several (including 0) students via advisor
• Many-to-many relationship


  • An instructor is associated with several (possibly 0) students via advisor
  • A student is associated with several (possibly 0) instructors via advisor
• Candidate key


  A minimal super key of an entity set
• Candidate keys


  • ID is candidate key of instructor
  • course_id is candidate key of course
• Primary key


  A candidate key that is most appropriate to be the main reference key for the table. It must contain unique values, must never be null and uniquely identify each record in the table
• Foreign key


  A primary key from one table that appears as a field in another where the first table has a relationship to the second
• Super key


  A set of one or more attributes whose values uniquely determine each entity
• Super keys


  • {Student ID, FirstName}
  • {Student ID, LastName}
  • {Student ID, FirstName, LastName}
• Rectangles represent entity sets, diamonds represent relationship sets, attributes listed inside entity rectangle or as oval shape along with the rectangle, underline indicates primary key attributes
• Crows foot notation is a common method of indicating cardinality, using symbols like single bar, double bar, circle, and crows foot
• A weak entity set does not have a primary key, its discriminator is underlined with a dashed line, and its identifying relationship is in a double diamond
• Generalization is a bottom-up approach where lower level entities combine to form a higher level entity, specialization is the opposite top-down approach
• Aggregation is when a relationship between two entities is treated as a single entity
• UML class diagrams correspond to ER diagrams, with some differences in notation